306 lines (226 sloc) 14.3 KB

Contributing to Clippy

Hello fellow Rustacean! Great to see your interest in compiler internals and lints!

First: if you're unsure or afraid of anything, just ask or submit the issue or pull request anyway. You won't be yelled at for giving it your best effort. The worst that can happen is that you'll be politely asked to change something. We appreciate any sort of contributions, and don't want a wall of rules to get in the way of that.

Clippy welcomes contributions from everyone. There are many ways to contribute to Clippy and the following document explains how you can contribute and how to get started. If you have any questions about contributing or need help with anything, feel free to ask questions on issues or visit the #clippy IRC channel on or meet us in #wg-clippy on Discord.

All contributors are expected to follow the Rust Code of Conduct.

Getting started

High level approach:

  1. Find something to fix/improve
  2. Change code (likely some file in clippy_lints/src/)
  3. Run cargo test in the root directory and wiggle code until it passes
  4. Open a PR (also can be done between 2. and 3. if you run into problems)

Finding something to fix/improve

All issues on Clippy are mentored, if you want help with a bug just ask @Manishearth, @llogiq, @mcarton or @oli-obk.

Some issues are easier than others. The good first issue label can be used to find the easy issues. If you want to work on an issue, please leave a comment so that we can assign it to you!

Issues marked T-AST involve simple matching of the syntax tree structure, and are generally easier than T-middle issues, which involve types and resolved paths.

T-AST issues will generally need you to match against a predefined syntax structure. To figure out how this syntax structure is encoded in the AST, it is recommended to run rustc -Z ast-json on an example of the structure and compare with the nodes in the AST docs. Usually the lint will end up to be a nested series of matches and ifs, like so.

E-medium issues are generally pretty easy too, though it's recommended you work on an E-easy issue first. They are mostly classified as E-medium, since they might be somewhat involved code wise, but not difficult per-se.

T-middle issues can be more involved and require verifying types. The ty module contains a lot of methods that are useful, though one of the most useful would be expr_ty (gives the type of an AST expression). match_def_path() in Clippy's utils module can also be useful.

Writing code

Llogiq's blog post on lints is a nice primer to lint-writing, though it does get into advanced stuff. Most lints consist of an implementation of LintPass with one or more of its default methods overridden. See the existing lints for examples of this.

If you want to add a new lint or change existing ones apart from bugfixing, it's also a good idea to give the stability guaratees and lint categories sections of the Clippy 1.0 RFC a quick read.

Author lint

There is also the internal author lint to generate Clippy code that detects the offending pattern. It does not work for all of the Rust syntax, but can give a good starting point.

First, create a new UI test file in the tests/ui/ directory with the pattern you want to match:

// ./tests/ui/
fn main() {
    let arr: [i32; 1] = [7]; // Replace line with the code you want to match

Now you run TESTNAME=ui/my_lint cargo test --test compile-test to produce a .stdout file with the generated code:

// ./tests/ui/my_lint.stdout

if_chain! {
    if let ExprKind::Array(ref elements) = stmt.node;
    if elements.len() == 1;
    if let ExprKind::Lit(ref lit) = elements[0].node;
    if let LitKind::Int(7, _) = lit.node;
    then {
        // report your lint here

If the command was executed successfully, you can copy the code over to where you are implementing your lint.


Please document your lint with a doc comment akin to the following:

/// **What it does:** Checks for ... (describe what the lint matches).
/// **Why is this bad?** Supply the reason for linting the code.
/// **Known problems:** None. (Or describe where it could go wrong.)
/// **Example:**
/// ```rust
/// // Bad
/// Insert a short example of code that triggers the lint
/// // Good
/// Insert a short example of improved code that doesn't trigger the lint
/// ```

Once your lint is merged it will show up in the lint list

Running test suite

Use cargo test to run the whole testsuite.

If you don't want to wait for all tests to finish, you can also execute a single test file by using TESTNAME to specify the test to run:

TESTNAME=ui/empty_line_after_outer_attr cargo test --test compile-test

Clippy uses UI tests. UI tests check that the output of the compiler is exactly as expected. Of course there's little sense in writing the output yourself or copying it around. Therefore you should use tests/ui/ (after running cargo test) and check whether the output looks as you expect with git diff. Commit all *.stderr files, too.

Running rustfmt

Rustfmt is a tool for formatting Rust code according to style guidelines. The code has to be formatted by rustfmt before a PR will be merged.

It can be installed via rustup:

rustup component add rustfmt

Use cargo fmt --all to format the whole codebase.

Testing manually

Manually testing against an example file is useful if you have added some println!s and test suite output becomes unreadable. To try Clippy with your local modifications, run env CLIPPY_TESTS=true cargo run --bin clippy-driver -- -L ./target/debug from the working copy root.

How Clippy works

Clippy is a rustc compiler plugin. The main entry point is at src/ In there, the lint registration is delegated to the clippy_lints crate.

clippy_lints/src/ imports all the different lint modules and registers them with the rustc plugin registry. For example, the else_if_without_else lint is registered like this:

// ./clippy_lints/src/

// ...
pub mod else_if_without_else;
// ...

pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
    // ...
    reg.register_early_lint_pass(box else_if_without_else::ElseIfWithoutElse);
    // ...

    reg.register_lint_group("clippy::restriction", vec![
        // ...
        // ...

The rustc_plugin::PluginRegistry provides two methods to register lints: register_early_lint_pass and register_late_lint_pass. Both take an object that implements an EarlyLintPass or LateLintPass respectively. This is done in every single lint. It's worth noting that the majority of clippy_lints/src/ is autogenerated by util/dev update_lints and you don't have to add anything by hand. When you are writing your own lint, you can use that script to save you some time.

// ./clippy_lints/src/

use rustc::lint::{EarlyLintPass, LintArray, LintPass};

// ...

pub struct ElseIfWithoutElse;

// ...

impl EarlyLintPass for ElseIfWithoutElse {
    // ... the functions needed, to make the lint work

The difference between EarlyLintPass and LateLintPass is that the methods of the EarlyLintPass trait only provide AST information. The methods of the LateLintPass trait are executed after type checking and contain type information via the LateContext parameter.

That's why the else_if_without_else example uses the register_early_lint_pass function. Because the actual lint logic does not depend on any type information.

Fixing build failures caused by Rust

Clippy will sometimes fail to build from source because building it depends on unstable internal Rust features. Most of the times we have to adapt to the changes and only very rarely there's an actual bug in Rust. Fixing build failures caused by Rust updates, can be a good way to learn about Rust internals.

In order to find out why Clippy does not work properly with a new Rust commit, you can use the rust-toolstate commit history. You will then have to look for the last commit that contains test-pass -> build-fail or test-pass -> test-fail for the clippy-driver component. Here is an example.

The commit message contains a link to the PR. The PRs are usually small enough to discover the breaking API change and if they are bigger, they likely include some discussion that may help you to fix Clippy.

To check if Clippy is available for a specific target platform, you can check the rustup component history.

If you decide to make Clippy work again with a Rust commit that breaks it, you probably want to install the latest Rust from master locally and run Clippy using that version of Rust.

You can use rustup-toolchain-install-master to do that:

cargo install rustup-toolchain-install-master
rustup-toolchain-install-master -n master --force
rustup override set master
cargo test

Issue and PR triage

Clippy is following the Rust triage procedure for issues and pull requests.

However, we are a smaller project with all contributors being volunteers currently. Between writing new lints, fixing issues, reviewing pull requests and responding to issues there may not always be enough time to stay on top of it all.

Our highest priority is fixing crashes and bugs. We don't want Clippy to crash on your code and we want it to be as reliable as the suggestions from Rust compiler errors.

Bors and Homu

We use a bot powered by Homu to help automate testing and landing of pull requests in Clippy. The bot's username is @bors.

You can find the Clippy bors queue here.

If you have @bors permissions, you can find an overview of the available commands here.


Contributions to Clippy should be made in the form of GitHub pull requests. Each pull request will be reviewed by a core contributor (someone with permission to land patches) and either landed in the main tree or given feedback for changes that would be required.

All code in this repository is under the Apache-2.0 or the MIT license.